1. Field of the Invention
The present invention relates to a vehicle and, in particular, to a vehicle having a front frame supporting a front wheel and a rear frame supporting a rear wheel.
2. Description of Related Art
Some conventional vehicles, such as motorcycles, include a front frame supporting a front wheel and a rear frame supporting a rear wheel. The front and rear wheels of one type of conventional motorcycle include a round tire having an arcuate-shaped portion that contacts the ground. This type of conventional motorcycle has a drawback such that the vehicle body can incline sideways and stagger when it runs at an extremely slow speed, or the vehicle body also can incline sideways unless using any tumble preventing means such as, for example, a stand or an auxiliary wheel when the vehicle body is standstill.
Another type of conventional motorcycle, as disclosed in Japanese Patent Document JP-A-Sho63-222987, includes a front wheel with a round tire and a rear wheel having a “flat tire” with a U-shaped cross-section, wherein the portion that contacts the ground is flatly shaped. This motorcycle includes a structure in which a swingable shaft positioned between a swing vehicle body supports the front wheel and a non-bank vehicle body supports the rear wheel. The front portion of the vehicle body (swing vehicle body) is swingable under a condition that the rear portion of the vehicle body (non-bank vehicle body), that includes an engine, extends upright. That is, the front wheel can incline relative to a vertical direction, while the rear wheel stands upright relative to the ground regardless of vehicle speed. In this motorcycle, the stagger or tumble of the vehicle body can be controlled to a certain extent, if a rider operates the front wheel to not incline relative to the vertical direction in an extremely slow speed running state or in a standstill state.
However, in this conventional motorcycle disclosed in JP-A-Sho63-222987, when a rider adds weight to the motorcycle by sitting on the front portion (swing vehicle body) that is swingable relative to the rear wheel, the centroid is considered to shift to the inclination direction of the front portion (swing vehicle body) if the front wheel inclines relative to the vertical direction when the motorcycle runs at the extremely slow speed. Accordingly, there exists a problem that sufficient control of the stagger of the vehicle body is difficult. Also, when the front wheel starts inclining relative to the vertical plane while the motorcycle is standstill without any rider, it is difficult to control the inclination of the front wheel. Thus, there arises another problem that the vehicle body tumbles over unless a tumble preventing means is used, such as a stand, an auxiliary wheel, or the like.